Liquid crystal displays (LCDs) are used in a wide variety of modern electronic and computing devices including computer monitors, laptop computers, smart phones, handheld gaming systems and media players (portable audio players, portable video players, etc.). The widespread adoption of LCDs is due, in part, to their lower size, weight and power consumption relative to other types of displays, such as cathode ray tube (CRT) displays.
FIG. 1 shows a perspective, exploded view of various layers of a simplified conventional liquid crystal display 100. The LCD 100 includes a backlight 110, horizontal and vertical polarizing filters 120 and 130, layers of electrodes 140 and 150 and a liquid crystal layer 160. The LCD 100 includes multiple picture elements, or pixels (display pixels), that are individually controllable to display an image. The LCD 100 individually controls each pixel to control the amount of source light produced by the backlight 110 that passes through both the horizontal polarizing filter 120 and the vertical polarizing filter 130.
The backlight 110 can include one or more LEDs (light emitting diodes), electroluminescent panels or other types of light source, and can include a layer of material to diffuse light from the light source(s). The backlight 110 is capable of producing source light at varying levels of intensity for the LCD 100 overall or, in some newer designs, for different areas of the LCD 100. The term “backlight level” refers to the intensity of the source light produced by the backlight 110.
The source light produced by the backlight 110 is typically unpolarized. If light is visualized as a waveform extending along an axis, the polarization of the light is the orientation of the waveform (e.g., horizontal, vertical, or at some other angle) relative to the axis. Unpolarized light is a jumble of different polarizations. The horizontal polarizing filter 120 permits horizontally polarized light to pass through but blocks other light. The vertical polarizing filter 130 permits vertically polarized light to pass through but blocks other light. Without the liquid crystal layer 160, all light from the backlight 110 would be blocked by the series of two polarizing filters 120, 130. In the liquid crystal layer 160, however, molecules of liquid crystal at the position of a display pixel “twist” the light passing through such that horizontally polarized light becomes vertically polarized as it moves away from the backlight 110. The vertically polarized light can then pass through the vertical polarizing filter 130.
The degree of horizontal-to-vertical polarization change at a display pixel of the LCD 100 can be controlled by changing the amount of electrical current flowing between electrodes 140, 150 at the display pixel. The amount of light allowed to pass through the LCD 100 for each display pixel is thus determined by the amount of electrical current applied across the electrodes 140 and 150 at the pixel, where the amount of current applied can be controlled depending on desired pixel value intensity. Generally, the more intense, or brighter, the pixel value, the greater the amount of source light that is allowed to pass through the LCD 100 at the corresponding display pixel. For example, in one type of conventional LCD, nematic molecules twist light from horizontal to vertical polarization when the nematic molecules are in a “relaxed” state in which no current is applied, which permits light to pass through the vertical polarizing filter 130. The nematic molecules realign along the direction of current flow, however, when current is applied across the electrodes 140, 150. The degree of realignment depends on the strength of the current, permitting different amounts of light to pass through the vertical polarizing filter 130. When enough current is applied, the polarization of the horizontally polarized light is unchanged at the liquid crystal layer 160, so that the light is blocked by the vertical polarizing filter 130.
An image or frame to be displayed on the LCD 100 includes multiple pixels (image pixels) that have one or more pixel values and are associated during the display process with corresponding ones of the multiple display pixels on the LCD 100. For example, a single image pixel can have three pixel values corresponding to red, green and blue intensities, respectively, and the three pixel values can be combined to form any of a wide range of colors for the image pixel. On the LCD 100, a display pixel can include red, green and blue sub-pixels that are individually controlled by electrodes 140, 150 to set different intensities depending on the pixel values of the image pixel.
Different kinds of LCDs can use different mechanisms to control electrodes, different varieties of liquid crystals and different types of light sources for the backlight. Although LCDs consume less power than other types of displays, they can account for a large portion of the power consumed by computing devices, and the backlight uses much of the power consumed by a LCD. Thus, it is desirable, particularly for portable electronic devices, to reduce LCD power consumption to extend battery life and also save energy. LCD power consumption can be reduced by decreasing the intensity of the source light produced by the backlight 110. However, with reduced backlight intensity, the brightness of a displayed image is reduced, which can cause detail to be lost or otherwise hurt image quality as perceived by the viewer. Image quality as perceived by the viewer can also suffer in certain ambient light conditions. If the level of ambient level increases, a displayed image can be more difficult for a user to view, or a user can perceive less detail in the image.
Thus, there is a need for efficient, effective ways to enhance images to be displayed on a liquid crystal display in response to changes in backlight and ambient light levels.